Self-aligning phase conjugate laser

ABSTRACT

A method and apparatus are disclosed for providing a laser beam that is automatically aligned with a substantially rigid, stabilized platform or frame that can be oriented over a wide angular range, such as by the gimbals of a laser pointing and tracking system. A single-transverse-mode master laser oscillator 12 is mounted on the stabilized platform 13 which is part of the inner gimbal, which can be rotated about an elevation axis 16, and a multipass laser amplifier 21 wiht a phase conjugation mirror 22 and an optional nonlinear frequency-conversion device 20 are located off the inner gimbal. An outer gimbal or pedestal mount permits rotation about an azimuthal axis 17. The laser oscillator 12 and laser amplifier 21 are coupled by means of a beamsplitter 15 and two reflecting elements 18 and 19. The laser media used for the oscillator 12 and amplifier 21 are either the same, or compatible media having the same wavelength. In an alternative embodiment the two reflecting elements are replaced by a flexible ligh waveguide such as a glass fiber. The phase conjugation mirror 22 compensates the beam for the effects of optical aberrations caused by thermally induced changes in the amplifier medium and the nonlinear medium (if used) and also compensates the beam for angular tilt and jitter in the beam line of sight due to structural flexibility and motion of the stabilized platform. Four different embodiments are described in which the phase conjunction mirror is based on stimulated Brillouin scattering, degenerate four-wave mixing, three-wave mixing, and photon echo effects, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high-energy lasers used with pointingand tracking systems, and to precision laser power delivery systems.More particularly, the present invention relates to laser systems forproducing an automatically aligned beam that is compensated fordistortions in optical media and corrected for angular tilt and jitterin the beam line of sight due to gimbal motion and structuralcompliance.

2. Background Information

Many applications of laser systems demand precise control of thedirection and wavefront profile of the laser beam. A wavefront is athree-dimensional surface of constant phase, at right angles everywhereto a family of rays. Typical aberrations in the profile of the wavefrontinclude ones that alter the phase, focus, or astigmatic characteristicsof the beam. Control of these distortions and the line of sight of thebeam are of paramount importance in many applications involvinglong-distance communications, target ranging, the guidance of weaponssystems, and the delivery of laser power to a remote location.

Laser pointing and tracking systems that employ off-gimbal laser devicesin conjunction with gimballed tracking sensors are susceptible topointing errors in the line of sight resulting from (1) wander in thelaser beam due to thermal refractive effects and optical bench flexurein the laser itself, (2) static flexure and dynamical motion caused bythermal stresses and vibrations, respectively, and (3) angular wander ofthe line of sight due to bearing runout and nonorthogonality of thegimbal axes. "Wander" in a laser beam refers to changes in position ofthe centroid of a laser beam profile. Bearing "runout" refers to severalrelated phenomena which have to do with the fit of the bearing race orraces to a rotating shaft; for example, radial runout refers to theradial free play of the shaft in the bearing race or races, which allowsthe axis of the shaft to translate a certain amount parallel to itself,or to deviate from perpendicularity in its orientation with respect tothe plane of the bearing. Previous laser pointing and tracking systemshave attempted to control laser beam wander through good thermalmanagement and proper structural design, through the use of foldingelements such as corner cubes and roof prisms that make the alignmentinsensitive to changes in their positions, and more recently through theutilization of phase conjugation of the beam.

The use of phase conjugation techniques to correct laser beam wavefrontdistortion is known in the art and is used in order to take advantage ofthe benefits that result from its incorporation in laser systems. InU.S. Pat. No. 4,005,935, for example, Wang discloses a method andapparatus for providing a phase-compensated optical beam directed at aremotely located target. The effects of phase perturbations along thepath to the target are substantially cancelled, and neardiffraction-limited convergence of the beam on the target is obtained.

In U.S. Pat. No. 4,321,550, Evtuhov discloses a phase conjugationapparatus that corrects for optical distortion in high-power lasersystems, and minimizes optical components. His system for phaseconjugate correction is particularly suitable for use with an inertialconfinement nuclear fusion system.

In U.S. Pat. No. 4,233,571, Wang and Yariv disclose a laser thatself-corrects for distortions introduced into the laser output beamwavefronts by aberrations and time-varying phenomena internal to thelaser, such as vibration of the cavity reflecting surfaces, warping ofthe reflecting surfaces through heating, misalignment of the reflectingsurfaces, aberrations in the lasing medium, and turbulence in the lasingmedium (if the medium is not a solid). The self-correction of theeffects due to these causes allows higher system efficiency andperformance of the system at its diffraction limit, i.e., at its optimumfocusing capability.

Giuliano, in U.S. Pat. No. 4,429,393, discloses apparatus using phaseconjugation at two different frequencies in a laser ring resonator forthe purpose of providing a phase-compensated diffraction-limited outputbeam at either or both frequencies.

In U.S. Pat. No. 4,344,042, Hon discloses apparatus for aself-regenerative laser oscillator-amplifier that employs intracavityphase conjugation to provide compensation for optical inhomogeneities instrongly pumped laser media without suffering efficiency losses, inorder to achieve single-mode output with increased average and/or peakpower.

None of these inventions, however, directly addresses the problems ofmisalignment of the output beam of a gimballed laser system due tocompliance in the gimbal structure, imperfections or wear in the gimbalbearings, and nonorthogonalty of the gimbal axes. Presently the problemsof structural compliance and gimbal axis wander are controlled throughgood mechanical design and through the use of active input beamalignment systems. Typically the input beam alignment system is aclosed-loop servomechanical system that uses a collimated laser sourceand receiver to sense the angular deviation in pointing the beam. Theclosed-loop servomechanical system is utilized in combination with aprecision beam-steering mirror to provide vernier correction of thedisturbed line of sight. Typically such input beam alignment systems canbe quite complex, are limited in servo bandwidth because of reactiontorque feedback, and are themselves prone to misalignment. In U.S. Pat.No. 4,326,800, Fitts discloses such a complex system for laser beamwavefront and line-of-sight error correction. Fitts uses a low-energyreference beam at the vertex of a primary mirror that is grated todiffract a low-energy holographic replica of the high-energy primarybeam. A photodetector-based servo control system compares the line ofsight of the reference beam to that of the low-energy replica andgenerates control signals which actuate a beam steering mirror toreposition the main beam. The servo control system also includes awavefront sensor. The sensor analyzes the wavefront profile of thelow-energy replica and generates control signals which actuate adeformable mirror to correct spurious wavefront aberrations.

SUMMARY OF THE INVENTION

The self-aligning phase conjugate laser provides a high-power laser beamthat is automatically aligned with a stabilized platform mounted on theinner gimbal of a laser pointing and tracking system. The self-aligningphase conjugate laser uses an on-gimbal single-transverse-mode laseroscillator in conjunction with an off-gimble multipass laser poweramplifier and phase conjugation mirror (commonly referred to as a phaseconjugate mirror) to provide an output beam that preserves the beamquality and alignment stability of the oscillator beam. If an optionalnonlinear optical device is also used, the self-aligning phase conjugatelaser offers the capability of distortion-free frequency conversion,such as frequency doubling. The single-mode laser oscillator andhigh-power laser amplifier are optically coupled by means of abeamsplitter and two reflective folding elements. In an alternativeembodiment the two reflecting elements are replaced by a flexibleoptical waveguide device such as a glass or plastic fiber that utilizestotal internal reflection to "pipe" the light to a target location. Thephase conjugation mirror in the configuration described abovecompensates for both the optical aberrations in the beam caused bydistortions in the amplifier medium (and frequency-conversion medium, ifused) as well as angular tilt and jitter in the beam line of sight dueto motion of the gimballed platform and compliance in its structure. Thephase conjugation mirror can take on several different forms, such asdevices that make use of stimulated Brillouin scattering, degeneratefour-wave mixing, three-wave mixing, and photon echo effects.

The undesirable features of active input beam alignment systems areavoided by the passive, self-aligning phase conjugation laser systemdisclosed in the present invention, which provides extremelywide-bandwidth compensation of all beam wander and misalignment effects.The present invention eliminates the need for complex input beamalignment systems and relaxes structural design constraints on the laserand gimbal.

The self-aligning phase conjugate laser provides a high-power laser beamthat is automatically aligned with the gimballed platform of a laserpointing and tracking system. It is able to compensate for angulardeviations and jitter in the beam line of sight caused by gimbal motionand structural compliance. It eliminates the need for complexelectromechanical servo systems that are limited in response bandwidthby reaction torque feedback and are themselves prone to misalignment. Inaddition, use of the self-aligning phase conjugate laser relaxes thestiffness constraints on the laser and gimbal structures.

The subject invention thus represents a completely new approach to beamalignment for laser pointing and tracking systems. The advantages of thepassive, phase conjugation approach to autoalignment include (1)compensation of high-frequency jitter, (2) higher system reliabilitysince additional servomechanisms are not required, (3) decreased systemsize and weight, and (4) lower development and production costs. Becauseof the improved performance at reduced cost, a multitude of potentialapplications can be envisioned.

The versatility of application of the subject invention should beapparent from considering two possible applications on vastly differentsize scales. At one extreme, for example, the gimballed platform of thesubject invention might be on a satellite moving in orbit high above theearth's atmosphere, with the high-power laser amplifier being based onthe earth's surface. At the other extreme, a surgeon's hand might be theequivalent of the gimballed platform referred to in the presentinvention, with a high-power medical laser providing a beam through aflexible optical waveguide to be directed by the finger, hand, and armmuscles of the surgeon in the performance of some delicate and preciseoperation.

Accordingly, it is one object of the present invention to provide ahigh-power laser beam that is automatically aligned with the platform orframe of a pointing and tracking system, or with the directed end of anoptical fiber in a precision laser power delivery system.

It is another object of the present invention to provide a high-powerlaser beam capable of being directed over a wide angular range withoutthe use of complex and cumbersome closed-loop servomechanical input beamalignment systems.

It is yet another object of the present invention to provide ahigh-power gimbal-oriented laser beam that is compensated over anextremely wide bandwidth for all beam wander and misalignment effects,thus eliminating the need for strict stiffness constraints on the laserand gimbal structures.

An appreciation of other aims and objects, along with a more completeunderstanding of the present invention, may be achieved through thestudy of the following description of a preferred embodiment in additionto reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a schematic diagram showing the disposition of thevarious components of a preferred embodiment of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the figure, a preferred embodiment of the self-aligningphase conjugate laser 10 is shown, including a single-transverse-modelaser oscillator 12 mounted on a stabilized platform 13 which is part ofthe inner gimbal of a laser pointing and tracking system or precisionlaser power delivery system. The stabilized platform 13 serves as anoptical bench on which are also mounted a tracking sensor 14 and abeamsplitter 15. The tracking sensor 14 is typically a television cameratube or forward-looking infrared (FLIR) sensor to provide a means ofgenerating angular error signals to be used by servomechanical systemsto point the stabilized platform 13 located on the inner gimbal of thepointing and tracking system. Mounting the single-mode laser oscillator12, the track sensor 14, and the beamsplitter 15 on the samesubstantially rigid, stabilized platform 13 which constitutes part ofthe inner gimbal allows the minimization of misalignment errors andjitter arising from structural compliance. The high-quality laseroscillator beam exits the inner gimbal stabilized platform 13 byreflection off the beamsplitter 15, which is a partially reflectingplanar surface. After reflection off the beamsplitter 15 thehigh-quality single-transverse-mode laser oscillator beam passes throughthe structural framework (or its equivalent in the alternativeembodiment) comprising the gimbal axes 16 and 17 by reflections off tworeflecting elements 18, 19 and possibly some additional folding orrelaying reflective elements (not shown). In one possible embodiment thereflecting elements 18 and 19 are "full-angle" reflecting elements, sonamed because they are disposed to give an amount of angular deviationof a reflected beam equal to the angle through which the elementrotates. The reflecting element 18 is attached to the inner gimbal ofthe pointing and tracking system but is not part of the stabilizedplatform 13. The relecting element 19 is attached to an outer gimbal orpedestal mount, depending on the particular embodiment of the pointingand tracking system. There may also be additional reflecting elements tofold the optical path between reflecting elements 18 and 19. Thereflecting elements can be chosen from a variety of conventionalmetallized or dielectric-coated mirrors or total-internal-reflectionprisms designed to give maximum reflection over a range of wavelengthsthat includes the laser oscillator wavelength.

In an alternative embodiment, the two reflecting elements 18 and 19 arereplaced by a flexible optical waveguide such as a glass or plastic orother type of optically transmissive fiber which makes use of thephenomenon of total internal reflection to effect a "piping" of thelaser radiation from one end of the waveguide to the other. Thisalternative embodiment makes use of the flexible nature of an opticallytransmissive fiber to couple the light beams going back and forthbetween the stabilized platform 13 and the off-gimbal laser amplifier 21or nonlinear frequency conversion device 20. Such as alternativeembodiment would be particularly appropriate in cases where it isdesirable to deliver laser power to a relatively small target area, asin laser surgery or laser microfabrication techniques. In the case oflaser surgery, the hand of the surgeon would be the equivalent of thestabilized platform referred to in the first preferred embodiment.

For some applications the low-power, high-quality laser oscillator beamwill pass through a nonlinear frequency-converting device 20, but willnot undergo frequency conversion in any significant amount because ofthe relatively low energy density in the beam. A laser power amplifier21 comprising one or more segments (only one is shown) provides highgain at the oscillator wavelength. The wavefront of the laser beamemerging from the laser power amplifier 21 will be aberrated because ofthermal lensing in the medium of the laser power amplifier and also inthe medium of the nonlinear frequency-converting device 20 (if one ispresent). Aberrations may also be introduced by beam wander due toinhomogeneities in the power amplifier and frequency converter media.Furthermore, this same beam will be misaligned because of flexibility inthe structure associated with the gimbals, will wander because of wearor inherent manufacturing imperfections in the bearings, and will wanderin its line of sight because of nonorthogonality effects in the gimbalaxes, such as the gimbal axes not being orthogonal to one another.

The phase conjugation mirror 22 generates a counterpropagating beamwhose wavefront is the complex phase conjugate image of the incidentwavefront. The phase conjugation mirror 22 can have differentembodiments according to the mechanism whereby the nonlinear opticalmedium employed in it produces the phase conjugate image of the beamincident on it.

In one embodiment, the phase conjugation mirror 22 is a device based onthe effect known as stimulated Brillouin scattering. Acoustic waves areset up in a nonlinear optical medium such as pressurized methane,tetrafluoromethane, or carbon disulfide. Any nonlinear optical mediumwhich can be used for Brillouin scattering, whether it is a solid,liquid, or a gas, may be employed. The acoustic waves are producedthrough the process of electrostriction, which involves the interactionbetween the large electric field intensities present in the incidentbeam and the nonlinear scattering medium. The density of the scatteringmedium is periodically modulated by the electrostriction process, andacoustic waves are set up in response to the electric field. Thisprocess requires sufficient optical energy, such as can be furnished bya laser, because there is a power threshold below which stimulatedBrillouin scattering will not occur. The acoustic waves which aregenerated in the scattering medium are produced in a time interval onthe order of nanoseconds, which is extremely fast compared to the timesassociated with turbulence, thermal conduction, the propagation ofmechanical disturbances, and many allied phenomena. Within thestimulated Brillouin scattering medium, the acoustic waves that are setup conform identically to the incident optical wavefronts, and act asreflecting surfaces for the wavefronts that impinge on the acousticwaves. Light waves that impinge on the acoustic waves are reflected sothat their wavefronts become the complex phase conjugate image of theincident wavefronts. A frequency shift arising from the Doppler effectoccurs in the retroreflection of light waves from the receding acousticwaves in the stimulated Brillouin scattering medium. This shift is onthe order of 1 part in 100,000 and does not affect performance.

Therefore, when the phase-conjugated beam traverses the laser amplifier21, the nonlinear frequency-converting device 20, and the gimbal optics18 and 19, any any optical aberrations and angular misalignments of thebeam apparent after the first pass are perfectly compensated. After asecond pass through the laser power amplifier 21, the beam hassufficient intensity to be efficiently converted by the nonlinearfrequency-converting device 20 if one is used. The output beam 23, whichis outcoupled through the beamsplitter 15, exhibits the optical qualityand alignment accuracy of the on-gimbal laser oscillator 12 mounted onthe stabilized platform 13, regardless of any line-of-sight jitter oroptical distortion introduced by the amplifier medium, nonlinear medium,and the gimbal system.

If polarization-sensitive elements are employed, such as a nonlinearfrequency converting crystal for 20, a means of polarization de-rotationis required to compensate the polarization rotation caused by the motionof the reflecting elements 18 and 19. A pair of quarter-wave plates orrhomboidal prisms, one located between the beamsplitter 12 and thereflecting element 18 and the other located between the reflectingelement 19 and the nonlinear frequency converting device 20, willaccomplish the required polarization de-rotation.

The laser oscillator 12 and laser power amplifier 21 employ either thesame type of gain medium, or compatible types having the samewavelength, as a result of their gain curves overlapping at least inpart. Possible gain media may include a crystal, such as ruby orneodymium-doped yttrium aluminum garnet (YAG); a doped glass, such asneodymium-doped glass; a semiconductor, such as gallium arsenide; a gas,such as carbon dioxide; a liquid containing a fluorescent dye, such asrhodamine 6G; or other gain media known in the art. The gain medium ineither case (in laser oscillator 12 or in power amplifier 21) is excitedby an appropriate conventional means not shown, such as the light from axenon flashlamp, a high-voltage electrical discharge, a high-energyelectron beam, or another laser. An example of the use of compatiblegain media would be the use of a 1.06-micrometer laser diode such asindium gallium arsenide as the oscillator 12 located in the hand-heldportion of a laser surgical instrument, with a 1.06-micrometerneodymium:YAG or neodymium:glass laser amplifier 21 remotely located andcoupled to the hand-held portion of the instrument with a glass fiberwaveguide.

The phase conjugation mirror 22 may take on different forms in differentembodiments of the self-aligning phase conjugate laser 10 as claimedbelow. Besides the process of stimulated Brillouin scattering used inthe first embodiment described above, other embodiments may beenvisioned in which use is made of the phenomena commonly referred to asdegenerate four-wave mixing, three-wave mixing, and photon echo effects.All these phenomena are described in articles and books covering thetopic of nonlinear optics, such as, for example, the third edition ofthe book entitled Optical Electronics, written by Amnon Yariv andpublished in 1985 by Holt, Rinehart, and Winston in New York; thearticle "Nonlinear Optical Phase Conjugation," by D. M. Pepper, in TheLaser Handbook, volume 4, edited by M. Bass and M. Stitch and publishedby North-Holland in New York in 1985; and the book Optical PhaseConjugation by R. A. Fisher, published by Academic Press in New York in1985. Embodiments of phase conjugation mirrors utilizing three-wavemixing, degenerate four-wave mixing, and photon echo effects are knownin the art and are described in various U.S. Patents such as U.S. Pat.No. 4,321,550--Evtuhov and U.S. Pat. No. 4,233,571--Wang and Yariv.

The first embodiment of the phase conjugation mirror 22 is a stimulatedBrillouin scattering device, in which an incident wavefront that hasbeen deformed by some optical aberration sets up acoustic waves in asuitable medium such as pressurized methane, tetrafluoromethane, orcarbon disulfide. The acoustic waves are produced by electrostriction, aprocess in which the very large electric field intensitites in theincident laser beam interact with the medium. The density of the mediumis periodically modulated by the electrostriction process in a timeextremely small compared to that of any mechanism that may have causedthe distorted wavefronts of the incident light waves. The periodicdensity variations associated with the acoustic waves serve asreflecting surfaces for the aberrated wavefronts impinging on theacoustic waves. The complex phase conjugate image of the incidentoptical wavefront is reflected, and when the reflected wave reencountersthe aberration that initially caused the deformation, the distorted waveis corrected as it passes the aberration.

The second embodiment of the phase conjugation mirror 22 employs theprocess of degenerate four-wave mixing to accomplish the wavefrontcorrection. Two pump waves, emitted by either two identical lasershaving the same wavelength or one laser in combination with abeamsplitter arrangement, produce coherent optical beams which areincident on a nonlinear medium from opposite directions. A phasehalogram is set up in the medium by the interaction of the two pumpwaves and an aberrated wavefront with the medium. The aberratedwavefront incident on the medium is reflected as the phase conjugatewaveform. Alternatively, an appropriate absorbing or amplifying mediumis used which results in amplitude holograms being established in themedium and this leads to the phase conjugation process.

The third embodiment makes use of the process of three-wave mixing,often referred to as parametric downconversion. The incident aberratedwavefronts strike a nonlinear medium, and in addition, an external laseremits waves of a pump frequency which is twice that of the aberratedwaves and which are also made incident on the medium from the samedirection. The interaction of the waves and the medium produces thephase conjugate waveform, which is propagated through the nonlinearmedium. This waveform is then transmitted back along the initial opticalpath of the incident aberrated wavefront by conventional means.

Yet another embodiment utilizes the process of photon echoes to producephase conjugate reflected wavefronts. This process is akin to stimulatedBrillouin scattering, except that the nonlinear medium is different. Inthe photon echo process, an incident aberrated wavefront deforms themedium. A laser pulse emitted by an external laser that impinges on thesame medium at a later time is reflected as the complex phase conjugatewaveform of the aberrated incident waveform. This process is extremelyfast, even faster than the process involving stimulated Brillouinscattering. The process takes place within several centimeters of thesurface of the nonlinear medium on which the aberrated wavefronts areincident.

The nonlinear frequency-converting device 20 can take on different formsaccording to the exact physical mechanism employed, although most ofthem rely on the nonlinear optical properties of certain media. Two suchmechanisms are frequency doubling, also known as second harmonicgeneration, and stimulated Raman scattering.

The explanation of frequency conversion effects in nonlinear opticalmedia lies in the way a beam of light propagates through a dielectricmedium. A material medium consists of atoms or molecules whose nucleiand associated electrons form electric dipoles. Electromagneticradiation in the form of a light beam interacts with these dipoles andcauses them to oscillate. These oscillating dipoles themselves act assources of electromagnetic radiation. If the amplitude of vibration ofthe dipoles is small, the radiation they emit has the same frequency asthat of the incident radiation. As the intensity of the incidentradiation increases, however, nonlinear effects eventually come intoplay which produce harmonics of the frequency of oscillation of thedipoles. The second and strongest frequency harmonic is at twice thefrequency of the incident radiation. Not all solids exhibit frequencydoubling; the phenomenon is not observed for solids that have a centerof symmetry in their structure. In crystals which do producefrequency-doubled light, dispersion causes the frequency-doubled lightto travel at a different velocity than the light whose frequency is notdoubled. Destructive interference effects result in periodic variationsin the intensity of the frequency-doubled light through the crystal. Ifthe speeds of propagation of the beams can be made equal, a morepowerful frequency-doubled beam is obtained. A technique for speedequalization, also known as phase matching, can be achieved usingbirefringent crystals for which the dispersion is less than thebirefringence. Crystals of ammonium dihydrogen phosphate (ADP) andpotassium dideuterium phosphate (KD*P) belong to this group of materialsand are commonly used for second harmonic generation in commercial lasersystems, where efficiencies of 20 to 30 percent have been achieved.Several new materials present the possibility of higher conversionefficiency. Lithium niobate yields a high conversion efficiency but hasan index of refraction which depends strongly on laser power. Thiseffect is known as optical damage and in lithium niobate is known not tooccur above 160 degrees Centigrade. Frequency doublers employing lithiumniobate must be kept in an oven with an accurately controlledtemperature for phase matching. Another material, barium sodium niobate,has an even higher frequency conversion efficiency and does not appearto suffer from optical damage.

Another effect that can be used in frequency conversion is stimulatedRaman scattering. In the ordinary Raman effect a photon of an incidentlight beam is scattered by a molecule and emerges with a differentwavelength. For a monochromatic beam, there will be more than oneshifted spectral line, in general. If an emitted line has a wavelengthlonger than that of the incident beam, it is called a "Stokes line." Anemitted spectral line with a wavelength shorter than that of theincident beam is called an "anti-Stokes line." The difference in energybetween the emitted and incident photons is due to changes invibrational, electronic, spin, and rotational levels of the molecule,with a decrease in energy corresponding to the Stokes line and anincrease in energy corresponding to the anti-Stokes line. The scatteredbeams at particular wavelengths appear in well-defined cones about thedirection of the incident beam. In stimulated Raman scattering thephotons emitted in the ordinary Raman effect are made to stimulatefurther Raman emissions. With strong pumping of a Raman-active medium bya laser, gain can achieved at the wavelengths corresponding to theStokes and anti-Stokes spectral lines. This pumping can be used to setup oscillations at these wavelengths. Hydrogen, deuterium, and methaneare some of the molecular gases which have been used, usually underpressure because the effect is enhanced with increased density of thegas.

Experimental results demonstrate that a passive phase conjugation mirroreffectively compensates for laser beam wander. A Nd:YAG laser pulsed at5 Hertz, a scanning prism to induce beam wander, and a simple phaseconjugation mirror comprising a focusing lens and a cell containingpressurized methane were used to show that a root-mean-square beamwander as large as approximately 4 milliradians is reduced to theapproximately 10-microradian residual beam wander of the original laser.Furthermore, this compensation capability of nearly three orders ofmagnitude (a factor of 1000, or ten raised to the third power) does notconstitute a fundamental limit. Compensation for even greater beamwander was not attempted because the amount of compensation achieved isestimated to exceed the requirement for beam wander compensation thatmight be encountered in practical laser oscillator-amplifier systems ofhigh average power. Results were also obtained showing that the phaseconjugate mirror compensates for aberrations that might exist in theoptical path. Using a poor-quality optical element in conjunction with aconventional mirror increased the beam divergence by a factor of three,but produced a negligible increase in beam divergence of less than 10percent when used with the phase conjugate mirror.

Although the present invention has been described in detail withreference to a particular preferred embodiment, persons having ordinaryskill in the art will appreciate that various modifications andalterations may be made without departing from the spirit and scope ofthe invention.

What is claimed is:
 1. Apparatus for automatically aligning a beam ofradiation with a pointing device comprising:a radiation emission meanswhich is mechanically, fixedly coupled to said pointing device, suchthat substantially all rotation and translation of said pointing deviceis simultaneously and synchronously experienced by said radiationemission means; nonlinear phase conjugate reflecting means which ismechanically decoupled from said pointing device; and radiation couplingmeans disposed between said emission means and said phase conjugatereflecting means for conveying emitted radiation from said emissionmeans to said phase conjugate reflecting means such that any aberrationinduced into said emitted beam by a motion of said pointing device issubstantially compensated for by said phase conjugate reflecting means.2. Apparatus as claimed in claim 1 in which said radiation emissionmeans is a laser.
 3. Apparatus as claimed in claim 1 in which saidpointing device is a tracking sensor which scans for an externalradiation beam and locks on to said external radiation beam once saidexternal radiation beam is sensed.
 4. Apparatus as claimed in claim 1 inwhich said radiation emission means and said pointing device are rigidlyand mechanically connected to a common frame.
 5. Apparatus as claimed inclaim 4 in which said common frame is an inner gimbal of a laser weaponsystem.
 6. A laser system comprising:a pointing and tracking sensormounted on a movable gimbal, said sensor being capable of being alignedwith a remote target; a laser oscillator capable of emitting a coherentbeam, said laser being mounted on said movable gimbal; focusing meansfor steering said coherent beam to said remote target; and laseramplifier means for amplifying said coherent beam, automaticallyaligning said coherent beam, controlling beam wander, and compensatingfor high frequency jitter, said laser amplifier means being mounted offof said movable gimbal and wherein said focusing means is operable forconveying at least a portion of said coherent beam to said laseramplifier means.
 7. A laser system as claimed in claim 6 in which saidlaser oscillator is a single transverse mode laser oscillator.
 8. Alaser system as claimed in claim 6 in which said focusing means includesa beamsplitter and a plurality of folding mirrors.
 9. A laser system asclaimed in claim 6 in which said focusing means includes a polarizingoutput coupler.
 10. A laser system as claimed in claim 6 in which saidfocusing means includes a flexible optical waveguide.
 11. A laser systemas claimed in claim 6 in which said laser amplifier means furtherincludes a phase conjugation means for amplifying said coherent beam.12. Apparatus as claimed in claim 11 in which said phase conjugationmeans employs Brillouin scattering for amplifying said coherent beam.13. Apparatus as claimed in claim 11 in which said phase conjugationmeans employs parametric downconversion for amplifying said coherentbeam.
 14. Apparatus as claimed in claim 11 in which said phaseconjugation means employs four-wave mixing for amplifying said coherentbeam.
 15. Apparatus as claimed in claim 11 in which said phaseconjugation means employs photon echo effects for amplifying saidcoherent beam.
 16. Apparatus as claimed in claim 6 which furtherincludes a frequency conversion means.
 17. Apparatus as claimed in claim16 in which said frequency conversion means is a frequency doublingdevice.
 18. Apparatus as claimed in claim 17 in which said frequencyconversion means employs stimulated Raman scattering.
 19. A tracking andlaser fire control system for finding a target and firing a laser beamat said target comprising:a movable gimbal mounted on a platform; apointing and tracking sensor mounted on said movable gimbal; a singletransverse mode laser oscillator mounted on said movable gimbal;focusing means mounted on said movable gimbal for pointing said laserbeam at said remote target; a nonlinear frequency converter mounted onsaid platform; a laser amplifier mounted on said platform; a passivephase conjugation cell mounted on said platform; said sensor beingcapable of receiving external radiation, comparing said externalradiation to a plurality of signal characteristics stored electronicallyin said sensor, and transmitting an activation signal to said laseroscillator if said received external radiation correlates positivelywith said plurality of stored signal characteristics in order to delivera high energy laser beam to said target simultaneously with saidtarget's acquisition by said sensor.
 20. A method for delivering laserenergy to a remote target including the steps of:placing a referencelaser oscillator on a common, stabilized, rigid, movable platform thatis controlled by a pointing mechanism together with a tracking sensormounted on said platform; and ensuring that an output beam originatingfrom said reference laser oscillator, passing through a plurality ofdirecting elements that are mounted off said platform, and passingthrough a laser amplifier mounted off said platform is aligned with aline-of-sight direction that corresponds to a target direction dictatedby said tracking sensor mounted on said platform by employing a timereversal technique that reverses the direction of propagation of anaberrated wavefront impinging upon a time reversal device; saidaberrated wavefront being generated by said reference laser oscillatorand a laser power amplifier mounted off said platform; whileconcomitantly preserving said impinging aberrated wavefront withoutinverting said aberrated wavefront relative to said time reversaldevice.